From pudding to Petri Dish

In the late 19th century, Lina Hesse, the American wife of a German scientist, recalled the advise of friends who had lived in Asia and made agar jellies and puddings that stayed solid despite the summer heat. Her husband relayed the message to his boss, microbiologist Robert Koch, who then used agar to isolate the causative agent of tuberculosis, Mycobacterium tuberculosis.

Agar gels are used as an invaluable tool in the microbiology laboratory. Scientists make agars containing various nutrients that help grow and differentiate bacteria. Agar is helpful to microbiologists because

• very few bacteria can digest the agar carbohydrates, so agar gels remain intact and bacteria colonies separate. Bacteria readily digest the proteins in gelatin and many liquify the gel.
• Agar gels remain solid at the ideal growth temperature for most bacteria, around 100 degrees F/38 degrees C. Gelatin melts at this temperature.